Various sheet accumulator systems have been employed for accumulating sheet material such as paper sheets, documents, and the like into stacks. Accumulators can function to stack sheets in a proper sequence. For example, sheets can be stacked in an “A to Z” sequence, wherein the top sheet in the stack is the first sheet in the sequence and the sheets following are in sequence to the bottom sheet, the last sheet in the sequence. Alternatively, sheets can be stacked by an accumulator in a “Z to A” sequence, wherein the top sheet in the stack is the last sheet in the sequence and the sheets following are in reverse sequence to the bottom sheet, the first sheet in the sequence. Mail insertion machines employ sheet accumulators to accumulate sheets in a stack prior to processing, such as binding, and placement into envelopes.
Mail insertion machines typically transport sheets one-by-one along a “stream” to an accumulator for stacking in an “A to Z” or “Z to A” sequence. Sheets are commonly transported to an accumulator along a stream in a “Z to A” sequence, wherein the first sheet transported is the last sheet in an ordered sequence comprising a document, and the second sheet is the second to last sheet in the ordered sequence. Subsequent sheets are transported in the reverse sequence until the last sheet, the first sheet in the ordered sequence, is transported.
Under-accumulators are known devices for receiving sheets transported in a stream and accumulating the sheets in a sequence one below the other (“under-accumulating”). The accumulation of sheets one below the other is known as under-accumulating. For example, an under-accumulator receiving sheets in a “Z to A” sequence stacks the sheets in a “Z to A” sequence. Conversely, for example, an under-accumulator receiving sheets in an “A to Z” sequence stacks the sheets in an “A to Z” sequence.
Examples of under-accumulators are disclosed in U.S. Pat. Nos. 6,203,006; 5,915,686; 5,794,931; 5,692,745; 5,647,587; 5,590,873; 5,244,200; and 5,147,092. A typical under-accumulator includes at least two driven belts which engage a sheet at its upper and lower surface, respectively. An under-accumulator can also include shaft-mounted pulleys for directing and driving the belts, two side guides which engage and guide the edges of sheets being transported by the belts, a mechanism for lifting a stack of accumulated sheets, a ramp for directing the sheet downward under the stack of sheets, and a sheet-restraining means for preventing the stacked documents from being fed by the belts until all sheets for a particular set have been under-accumulated.
Current under-accumulators are limited in the number of sheets that can be under-accumulated due to the increasing weight of the sheets as they are stacked. This limitation results because it becomes increasingly difficult to overcome the weight of accumulated sheets as the number of sheets stacked increases. The lifting mechanism of an under-accumulator has a limited ability to place another sheet underneath the stack due to the increasing weight of the stack. Typically, the number of sheets required to equal the weight limit of the under-accumulator is known. In this instance, the under-accumulator can include instructions to stop when the predetermined number of sheets has been accumulated.
A common method for under-accumulating a sheet set that exceeds the weight limitation of a single under-accumulator is to separately under-accumulate subsets of the sheet set with the under-accumulator and then manually stack the under-accumulated subsets together in the proper stack sequence. The under-accumulation of sheets in this manner is undesirable because it is time-consuming and laborious for an operator. Therefore, it is desired to provide a system and methods for automatically under-accumulating a set of sheets in a stack, wherein the weight of the stack can exceed the weight limit for an under-accumulator.